If one day in history had to be picked as the birth of paleontology, it might be the day in 1666
when two fishermen caught a giant shark off the coast of Livorno in Italy. The local duke ordered
that this curiosity be sent to Niels Stensen (better known as Steno), a Danish anatomist working at
the time in Florence. As Steno dissected the shark, he was struck by how much the shark teeth
resembled tongue stones, triangular pieces of rock that had been known since ancient times.

Today, most people would instantly wonder whether the tongue stones were giant petrified shark teeth,
but in 1666 such a presumption was a tremendous leap. Few could imagine how living matter could be turned
to stone, and beyond that, encased in solid rockespecially if the rock were well above sea level
and contained remnants of a marine organism. Fossils were instead thought to have fallen from the sky,
or to be sports of naturepeculiar geometrical shapes impressed on the rocks themselves.

Stenos drawing of a shark head helped him see that tongue stones were actually
fossil shark teeth (right).

From living tissue to stone
Steno made the leap and declared that the tongue stones indeed came from the mouths of once-living
sharks. He showed how precisely similar the stones and the teeth were. But he still had to account
for how they could have turned to stone and become lodged in rock. Naturalists of Stenos
day were becoming convinced that matter was composed of different combinations of tiny
corpuscleswhat today we would call molecules. Steno argued that the corpuscles
in the teeth were replaced bit by bit, by corpuscles of minerals. In this gradual process, the teeth
didnt lose their overall shape as they turned from tissue to stone.

These exposed rock layers nicely illustrate Stenos Law, with the youngest layers at the
top and the oldest at the bottom.

Stenos Law of Superposition
But how could fossils end up deep inside rocks? Steno studied the cliffs
and hills of Italy to find the answer. He proposed that all rocks and
minerals were originally fluid. Floating on the surface of the planet
long ago, they gradually settled out of the ocean and created horizontal
layers, with new layers forming on top of older ones. Molten rock sometimes
intruded into the layers, reaching the top and spreading out into a new
layer of its own. As the rocks formed, they could trap animal remains,
converting them into fossils and preserving them deep within their layers.
Those horizontal layers represent a time sequence with the oldest layers
on the bottom and the youngest on top, unless later processes disturbed
this arrangement. This ordering is now referred to as Stenos Law
of Superposition, his most famous contribution to geology.

Steno was not the only naturalist of his day to propose that fossils belonged to living
creatures. Leonardo da Vinci
and Robert Hooke, for example,
also took up the same view. But Steno pushed the idea much further. He argued for the first
time that fossils were snapshots of life at different moments in Earths history and
that rock layers formed slowly over time. It was these two facts that served as the pillars
of paleontology and geology in future centuries. And fossils ultimately became some of the key
evidence for how life evolved on Earth over the past four billion years.